Controlling the Viewport

After clipping and division by w, depth coordinates range from -1 to 1,
corresponding to the near and far clipping planes. depthRange specifies a
linear mapping of the normalized depth coordinates in this range to window
depth coordinates. Regardless of the actual depth buffer implementation,
window coordinate depth values are treated as though they range from 0
through 1 (like color components). Thus, the values accepted by depthRange
are both clamped to this range before they are accepted.

The initial setting of (0, 1) maps the near plane to 0 and the far plane to
1. With this mapping, the depth buffer range is fully utilized.

It is not necessary that the near value be less than the far value. Reverse
mappings such as (1, 0) are acceptable.

Controls the affine transformation from normalized device coordinates to
window coordinates. The viewport state variable consists of the coordinates
(x, y) of the lower left corner of the viewport rectangle, (in pixels,
initial value (0,0)), and the size (width, height) of the viewport. When
a GL context is first attached to a window, width and height are set to
the dimensions of that window.

Push the current matrix stack down by one, duplicating the current matrix,
excute the given action, and pop the current matrix stack, replacing the
current matrix with the one below it on the stack (i.e. restoring it to its
previous state). The returned value is that of the given action. Note that
a round-trip to the server is probably required. For a more efficient
version, see unsafePreservingMatrix.

Normal Transformation

If rescaleNormal contains Enabled, normal vectors specified with
normal are scaled by a scaling
factor derived from the modelview matrix. rescaleNormal requires that the
originally specified normals were of unit length, and that the modelview
matrix contains only uniform scales for proper results. The initial value of
rescaleNormal is Disabled.